math/polymake: Remove NATIVE option and add GitHub to MASTER_SITES

* Add GitHub as primary download site and upstream website as backup
* Remove NATIVE option, users wanting to apply CPU specific optimization
should use CPUTYPE instead using ports framework
* Use OPTIMIZED_CFLAGS option to define O3 optimization

PR:		273316
Reviewed by:	Philipp Ost <bsd@philippost.de> (maintainer)
Sponsored by:	Blinkinblox